1. Field of the Invention
The present invention relates to a novel thermo-setting adhesive resin composition which excels in corrosion resistance. More particularly, the invention relates to a thermo-setting adhesive composition which is suitable for bonding the hems of the outer and inner panels of the door, bonnet or trunk lid of an automobile, for instance.
2. Brief Description of the Prior Art
Structural adhesives are used for metal-to-metal bonding and should exhibit an adhesive strength equivalent to, or in excess of, the strength of the substrate metal when subjected to external forces, thereby forming a literally integral part of the metal. Such structural adhesives have recently been used widely in lieu of the earlier bonding means such as riveting, bolting, welding, etc. in aircraft, can manufacturing, electric and electronics industries. In the assembling of automotive bodies, for instance, such structural adhesives should display a positive anticorrosive property particularly in the process of joining press-formed outer and inner panels for the construction of side doors, bonnet, trunk and other component parts. Corrosion resistance is an especially important quality that is required of cars to be shipped to cold districts.
Thus, in the cold districts, it is common practice to apply to the road surface an antifreezing agent such as rock salt, calcium chloride, etc. for the prevention of freezing of the road surface by such an agent dramatically accelerates the corrosion of the car body steel sheets, thereby detracting from the appearance and durability of the cars. To cope with this problem, a rust-inhibiting pigment such as red lead and zinc chromate is usually incorporated in the structural adhesive. However, such pigments contain heavy metals (lead, chromium, etc.) and as such presents safety risks for man and domestic animals because of the toxicity of the metals. In order to overcome the problem, we conducted an intensive research and found that the incorporation of a special aluminum orthophosphate rust inhibiting pigment in an epoxy resin adhesive gives an adhesive composition having an excellent rust preventing effect without interfering with the adhesive function of the epoxy adhesive. The finding was followed by further engineering studies, which have resulted in the present invention. In the assembling of automotive body components, the usual practice comprises hemming the periphery of an outer panel inwardly with respect to the periphery of an inner panel and a water-tight seal is formed using an adhesive. In this procedure, the adhesive is applied in a linear pattern to the periphery of the inner panel and the two peripheries are welded together, for example by spot welding.
The structural adhesive heretofore used for this purpose is a one-component type thermo-setting epoxy resin composition containing a rust inhibiting pigment and a metal powder, e.g. aluminum powder, which is designed to impart a certain degree of electrical conductivity to the adhesive so as to permit spot welding and electrodeposition. However, with such an adhesive composition, the deposition of the coating in and around the adhesive-carrying area is poor and, moreover, the electrode reaction of aluminum or other metal powder results in the evolution of hydrogen gas which causes a multiplicity of pinholes in the adhesive layer. As a result, rust develops from the periphery of the adhesive-carrying area within 24 hours when tested by the salt spray method. This problem is serious as thinner sheet steel is specified for reducing the weight of cars. Therefore, there has been a keen demand for the development of a structural adhesive which is conducive to electrodeposition coating and possessed improved rust inhibiting quality. From this point of view, much research has been undertaken to provide a structural adhesive with anticorrosive property and an adequate degree of electrical conductivity necessary for electrodeposition but there has not been available a structural metal-to-metal adhesive satisfying the above-mentioned requirements. We discovered, under the circumstances, that an adhesive composition with remarkably improved rust inhibitory activity can be obtained by incorporating a rust inhibiting pigment comprising an aluminum orthophosphate and a zinc compound in a thermo-setting epoxy resin adhesive.
The present invention is, therefore, directed to a structural adhesive comprising an epoxy resin, a curing agent therefor, and a rust inhibiting pigment comprising an aluminum orthophosphate and a zinc compound.